What are HTTP Interceptors?

In Angular 1.x there was an option to register objects as HTTP Interceptors. You could then use those interceptors to perform different operations and transformations on all HTTP calls made by your application. This was a very powerful feature of the framework because not only allowed setting things like base path for the REST API endpoint, CSRF headers and many other things but also transforming or caching responses from the server.

So what’s different in Angular 2?

Well, after looking at the documentation you won’t be able to find any reference to interceptors in the new version of the framework. At least for now, interceptors are not currently supported out of the box. You could just use the classic way of doing things and just extend the HTTP class in Angular 2 but that’s not very pretty.

Introducing the XHRBackend

Angular 2 uses this class to create all HTTP connections. By taking advantage of the Angular’s great DI engine you could potentially extend the base XHRBackend class and provide our custom implementation to the application. By taking control of the creation of the HTTP connections you will be able to implement the classic Angular interceptors this way.

Let’s look at some code

Let’s start by creating our own XHRBackend.

MyXHRBackend.ts

import {XHRBackend, Request, XHRConnection, Response} from '@angular/http';
import {Observable} from 'rxjs';
import 'rxjs/add/operator/catch';
import 'rxjs/add/observable/throw';

export class MyXHRBackend extends XHRBackend {

  createConnection(request: Request): XHRConnection {
    let connection: XHRConnection = super.createConnection(request);
    // Before returning the connection we try to catch all possible errors(4XX,5XX and so on)
    connection.response = connection.response.catch(this.processResponse);
    return connection;
  }

  processResponse(response:Response){
    switch (response.status) {
      case 401:
        // You could redirect to login page here
        return Observable.throw('your custom error here');
      case 403:
        // You could redirect to forbidden page here
        return Observable.throw('your custom error here');
      case 404:
        // You could redirect to 404 page here
        return Observable.throw('your custom error here');
      default:
        return Observable.throw(response);
    }
  }

}

The above XHRBackend extension will catch all 401, 403 and 404 errors.
In order to tell Angular to use our implementation instead of the default class we will use Angular’s great DI features. We just need to add our custom class to the providers list in the application main module.

AppModule.ts

import {NgbModule} from '@ng-bootstrap/ng-bootstrap';
import {HttpModule, XHRBackend} from '@angular/http';
import {AppComponent} from './app.component';
import {MyXHRBackend} from './MyXHRBackend';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    HttpModule
  ],
  providers: [
    {provide: XHRBackend, useClass: MyXHRBackend}
  ],
  bootstrap: [AppComponent]
})
export class AppModule {
}

By providing our custom implementation of the XHRBackend, Angular’s HTTP class will use this implementation for all http calls. This means that if any http call encounters 401, 403 or 404 errors we can act accordingly in our application.